SH7713 Datasheet, PDF (264/904 Pages) Renesas Technology Corp – Renesas 32-Bit RISC Microcomputer SuperHTM RISC engine Family / SH7700 Series

Section 6 Cache
6.3.3 Prefetch Operation
Prefetch Hit: The LRU is updated to indicate that the hit way is the most recently hit way. The
other contents of the cache are not changed. Instructions and data are not transferred from the
cache to the CPU.
Prefetch Miss: Instructions and data are not transferred from the cache to the CPU. The way that
is to be replaced is shown in table 6.2. The other operations are the same as those for a read miss.
6.3.4 Write Access
Write Hit: In a write access in write-back mode, the data is written to the cache and no external
memory write cycle is issued. The U bit of the entry that has been written to is set to 1, and the
LRU is updated to indicate that the hit way is the most recently hit way. In write-through mode,
the data is written to the cache and an external memory write cycle is issued. The U bit of the
entry that has been written to is not updated, and the LRU is updated to indicate that the hit way is
the most recently hit way.
Write Miss: In write-back mode, an external write cycle starts when a write miss occurs, and the
entry is updated. The way to be replaced is shown in table 6.3. When the U bit of the entry which
is to be replaced by entry updating is 1, the cache-update cycle starts after the entry has been
transferred to the write-back buffer. Data is written to the cache and the U bit and the V bit are set
to 1. The LRU is updated to indicate that the replaced way is the most recently updated way. After
the cache has completed its update cycle, the write-back buffer writes the entry back to the
memory. Transfer is in 16-byte units. In write-through mode, no write to cache occurs in a write
miss; the write is only to the external memory.
6.3.5 Write-Back Buffer
When the U bit of the entry to be replaced in write-back mode is 1, the entry must be written back
to the external memory. To increase performance, the entry to be replaced is first transferred to the
write-back buffer and fetching of new entries to the cache takes priority over writing back to the
external memory. After the fetching of new entries to the cache completes, the write-back buffer
writes the entry back to the external memory. During the write-back cycles, the cache can be
accessed. The write-back buffer can hold one line of cache data (16 bytes) and its physical
address. Figure 6.3 shows the configuration of the write-back buffer.
Rev.1.50 Aug. 30, 2006 Page 224 of 860
REJ09B0288-0150